An apparatus of this general type is disclosed in EP-A-241,894 and its counterpart U.S. Pat. No. 4,825,702. In this apparatus each clamping member has a band (in practice, a steel band) which is in contact around the roller through 180.degree.. The test material is inserted into the gap between band and roller and clamped in place between these by the rotation of the roller and the band, the clamping length reaching at most half the periphery of roller.
With this apparatus continuous testing of strength properties is made possible. The test material is continuously drawn-off from its supply and fed intermittently to the driven clamping members during the testing operation. A higher test speed is reached than in such tensile-testing installations as the apparatus sold by Zellweger Uster AG under the trademark USTER TENSORAPID.
The testing of strength properties, that is, maximum tensile force (breaking strength) and elongation, was for a long time regulated by a plurality of national and international standards. A new standard DIN 53834, which only permits the so-called CRE-Principle (CRE=Constant Rate of Elongation, or, in other words, constant rate of deformation), was introduced in 1976. In this respect, reference is made to the publication USTER News Bulletin No 26, November 1978 "USTER Prufverfahren fur das leistungsstarke Textillabor" (USTER Test Methods for the efficient textile laboratory), Page 32 ff.
The apparatus described in EP-A-241,894 and U.S. Pat. No. 4,825,702, has the considerable disadvantage that it does not permit any testing according to the CRE-Principle owing to the fact that the rate of deformation greatly depends on the elongation. In fact the profile of the rate of deformation is sinusoidal. The rate of deformation increases from zero when the test material is clamped in place up to a maximum value at a 90.degree. angle of contact on the rollers and then reaches zero again at 180.degree.. Other disadvantageous properties of this known apparatus also exist. Ever slight soiling between roller and band impairs the measuring accuracy. Moreover, the clamping principle that is used does not permit any large tensile forces. The latter means restriction with regard to the test material.